1. Field of the Invention
This invention relates to dual frequency cavity backed slot antennas and, more specifically, to such antennas which can be accurately tuned for operation at both operating frequencies by adjustment made at a single accessible surface thereof.
2. Brief Description of the Prior Art
Dual frequency cavity backed slot antennas are multi-layer microstrip antennas that operate at two separate frequencies. Such antennas are mounted on a ground plane which has an opening around the edges having a width and length selected according to the desired frequency characteristics of the antenna. A first top resonant microstrip layer is aligned in the plane of the ground plane and has a width and length less than the opening in the ground plane. Feed throughs (probes) electrically connect the microstrip element to a feed network. A container formed of a bottom and two sidewalls surrounds the antenna. Separating the first top resonant microstrip element from a bottom ground plane is a second resonant microstrip element mounted parallel to the first top microstrip element and electrically coupled to the feed probes. The container is electrically connected to the ground plane. The radiation slot or separation is the difference in the dimensions of the resonant microstrip elements and the opening or edges of the ground plane. The radiation slot may be covered with a thin membrane or microwave absorber.
At each frequency, the antenna circuit described above has very high quality factor (Q) which yields a narrow bandwidth. Because of material and manufacturing process variations, the resonant frequency or frequencies may offset from the desired operating frequency or frequencies. This is not a problem for one of the two resonant frequencies since the top resonant microstrip circuit is readily accessible and can be tuned after assembly to its selected resonant frequency. However, the second element is not accessible and therefore cannot be tuned subsequent to manufacturing assembly. It is therefore apparent that there exists the need of a capability to fine tune the antenna to either or both resonant frequencies of the antenna after the manufacturing assembly is complete.
There is no known published prior art relating to tuning a dual frequency cavity backed slot antenna. While stacked microstrip patch antennas are known and, at first glance may appear to be similar to dual frequency cavity backed slot antennas, these antennas differ from each other very significantly. In the stacked patch antenna, the metallized area on the upper layer does not extend to the edge. Therefore, no slot is formed on the first circuit layer. The metallization on the first circuit layer is then similar to that on the second circuit layer. There is no conductive cavity. In addition, the stacked patch antenna is usually mounted in the host with its bottom side flush with the host surface. This results in an antenna which forms a protrusion on the host surface. In contrast, the cavity backed dual frequency slot antenna mounts in the host flush with the host upper surface, in a conformal manner therewith and is surrounded by a conductive cavity. There is no protrusion above the host surface.